Narrowing in on the Higgs boson

A collaboration between research groups has further refined the mass limits on …

In the early days of the twentieth century, particle physics was in its infancy, with only two types of particles known: the proton and the electron. From the 1930s on, physicists were inundated by a deluge of new particles. In his 1955 Nobel Prize acceptance speech, Willis Lamb quipped that "the finder of a new elementary particle used to be rewarded by a Nobel Prize, but such a discovery now ought to be punished by a $10,000 fine." As time went on, particle physicists discovered the eightfold way—particle physics' equivalent of the periodic table—which ultimately led to the "standard model" of particle physics. To date, every particle predicted by the standard model has been experimentally verified, with the exception of the elusive Higgs particle.

The Higgs particle is theorized to be the reason why some particles, such as the W and Z bosons, have mass, while other force carriers, like the photon, are massless. Previous work has put lower and upper limits on the mass of this particle. CERN'sLarge Electron-Positron collider has been unable to find the Higgs particle at masses below 114 GeV, setting this as a lower bound for the mass. Studies of the electroweak force suggest that the Higgs particle must weigh less than 185 GeV. Now, work by a collaboration of the DZero and CDF particle discovery groups (each of those are collaborations in their own right) has narrowed this window even further.

The discovery of the Higgs boson is widely believed to be one of the first major findings that will be made at the Large Hadron Collider, which resides on the Swiss-French border. The LHC was turned on last fall, but suffered a setback and had to be shut down for maintenance that ran into CERN's standard winter maintenance/shutdown period. With the LHC offline, the Higgs search has shifted to�ongoing research�that uses previously obtained data. A recent announcement from Fermilab�indicates that a range of masses can now be excluded from the search for the Higgs particle.

Using data that has been generated over the last several years by�Fermilab's Tevatron, a collaboration between DZero and CDF found that the Higgs particle cannot exist at masses between 160 and 170 GeV, given a 95 percent confidence level. If one relaxes the level of confidence, then this exclusion range expands to cover masses between about 157 and 181 GeV. �

The standard model gives an idea of how often researchers should observe Higgs processes in a given set of data. The teams looked for Higgs signatures in three types of subatomic reactions: gluon-gluon fusion, vector boson fusion, or via its association with other vector bosons (pp*?WH?WW+W- or pp*?WH/ZH). Unfortunately, there are a handful of other processes that would mimic the signature that one would expect from the Higgs particle's decay, although the standard model gives an idea of how often these would be observed as well.�

To increase the chance of seeing an actual Higgs event, the researchers from DZero and CDF have pooled their data, essentially doubling what either had alone. "By refining our analysis techniques and by collecting more and more data, the true Higgs signal, if it exists, will sooner or later emerge," said DZero co-spokesperson Darien Wood of Northeastern University.

Since the LHC is anticipated to produce its first collisions later this year, it may not be long before we obtain clear evidence of the existence of the Higgs boson. I tend to think that this would be one of the least exciting answers possible, though. The real scientific treat would be if the LHC and other colliders can't find any Higgs particles. This would mean that the standard model, one of the shining examples of the power of particle physics and a theory on which a lot of physics rests, might be wrong; we would have to go back to the drawing board and invent something new.�

While this is unlikely, seeing as how the experimentalists have found 60 of the 61 particles that appear within the standard model, it sure would make the next few years interesting, both for scientists in the field and interested third parties like science journalists.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.